Redefine typedef within local scope

Question

I need to customize a typedef statement depending on a boolean value a. How can I solve this?

#include <iostream>

template<typename my_type>
ClassA{...};

int main ()
{
    bool a = false;

    typedef int my_type;
    if (a == true)
    {
        typedef int my_type;
    }
    else
    {
        typedef double my_type;
    }

    typedef ClassA<my_type> my_type2;

}

Answer 1

Short answer: you can't. Types are a compile-time notion and cannot depend on run-time factors.

However, function calls can! So, move your logic into a function template and call it conditionally:

#include <iostream>

template <typename T>
void foo()
{
   const T b = T(1) / T(3);
   std::cout << "b = " << b << std::endl;
}

int main ()
{
    bool a = false;

    if (a == true)
       foo<int>();
    else
       foo<double>();
}

This could, of course, be finessed, possibly with a boost::variant and some visitors.

I have at least fixed your problem with integer division. Ish.